Although traditional shipbuilding uses steel for almost the whole structure, concrete has also been used in the past in a few vessels. But in the last years, the offshore industry has been discovering the great possibilities that concrete presents as compared to steel, and it is being used in more and more projects, such as floating LNG terminals and in the development of the MOB concept (Mobile Offshore Base), as shown on the picture.
Concrete present a series of advantages of a big importance when talking about a structure for permanent use in the middle of ocean. These advantages supported by examples in Ref. [1], are:
Lower maintenance costs.
- This fact is supported by studies of floating concrete docks back in the 1970’s, showing dramatic savings, requiring less than 10% the maintenance of similar all-steel docks.
- Sare and Yee report negligible repair and maintenance costs for the 19 pre-stressed concrete barges constructed in the Philippines during 1964-66 for Lusteveco. The average annual maintenance costs of the concrete barges are found to be about 1/3 compared to steel barges.
Lower fabrication costs.
- The fabrication cost of Yee’s barges showed a saving of 16 percent compared to that of steel.
Downtime of the structure.
- In Yee’s barges, in the period 1974 to 1975, the total downtime per floating barge per year for maintenance work was six days for the concrete structures. The similar steel barges had an average downtime of 24 days.
Longer life of the structure.
- In a concrete structure, there is not a significant additional cost related to extension of design life from for instance 30 years to 50 years or 70 years. One reason is the fact that reinforced and pre-stressed concrete is not sensitive to fatigue.
Better motion behavior.
- The motion characteristics of a concrete hull are typically better than for a steel floater designed for the same purpose. This conclusion can be drawn based on reports from ship captains (World War II ships and Yee’s barges), several studies and recently confirmed by both analyses and model testing for very large FPSO’s (BP Atlantic Frontier Stage 2 / Schiehallion, hull length 280 m). The generally larger mass and draught, result in improved motion characteristics.
In the table below summarized from a similar one in Ref. [2] for floating LNG terminals, we can see the already mentioned advantages (and even more) of concrete compared to the ones for steel:
| ADVANTAGES FOR CONCRETE HULLS | ADVANTAGES FOR STEEL HULLS |
| Reduced Down-Time due to Inspection | Fabrication in Existing Shipyards |
| Reduced Maintenance Costs | Potentially Lower First Cost for One Hull |
| Economies of Scale | Traditional Engineering |
| Good Impact Resistance | Traditional Construction |
| Low Center of Gravity/Good Station Keeping Behavior/Reduced Motions | More Steel Fabricators are Available |
| Excellent Fatigue Life | More Steel Designers are Available |
|
High Mass Moment of Inertia
|
Prestressing Not Required |
| Slower Thermal Response/Better Insulation | |
| Resistance to Fatigue and Crack Propagation | |
| Resistance to Buckling |
In conclusion, it is expected that concrete could be a good alternative to steel for using in the structures for colonizing the oceans.
References:
[1] Sandvik, Knut; Eie, Rolf; Advocaat, Jan-Diederik (of Aker Kvaerner Engineering & Technology AS); Godejord, Arnstein; O.Hæreid, Kåre; Høyland, Kolbjørn; Olsen, Tor Ole (of Dr.techn.Olav Olsen a.s). Offshore Structures – A new challenge. How can the experience from the marine concrete industry be utilized. Acapulco: XIV National Conference on Structural Engineering, 2004.
[2] Berner, Dale; Gerwick, Ben C. Large Floating Concrete LNG/LPG Offshore Platforms. U.C. Berkeley, 2000
I haven’t seen any practicality in using steel for a permanent ocean structure. It is very maintenance intensive with a short life. I’d rather not have any steel in the concrete other than fiber, though it may be necessary for pre-stress, would rather design to eliminate pre-stress by avoiding cantilevered structures with any kind of moment joint. Use 3D hyberbolic arches (vaulted platforms and ceilings) instead.
My material preference is an alkali resistant glass fiber reinforced foam concrete coated with a protective layer of cement. It would be interesting to use a flexible concrete like Ductal with fiber reinforcement and foam. A good example of architecture which eliminates moment connections and cantilevers in this manner is from Roger Dean, designer of the Yes album covers, http://www.rogerdean.com/architecture/home-for-life/home-for-life-gallery.html . Flying Concrete also shows vaulted design though it does use welded steel mesh for reinforcement, http://www.flyingconcrete.com/index.htm .
I envision casting these shapes in the water from neutrally bouyant fiber reinforced foam concrete, then raising them up in the water after removing the reusable molds and pumping out the floatation chambers.
I would suggest to go away from spar design and approach the Nkossa Barge design…
a good pdf file (summary) about floating concrete platforms is available at:
http://www.tekna.no/arkiv/NB/Norwegian%20Concrete/Offshore%20Structures.pdf
other references…
…References
[1] Morgan, R. G. Development of the concrete hull. “Concrete Afloat”, Proceedings of the
conference on concrete ships and floating structures organized by The Concrete Society in
association with the Royal Institution of Naval Architects and held in London on 3 and 4
March, 1977.
[2] Gloyd, C. S. Concrete Floating Bridges. Concrete International, May 1988.
[3] Anderson, A. R. Design and Construction of a 375.000 bbl Prestressed Concrete Floating LPG
Storage Facility for the JAVA Sea. Offshore Technology Conference, OTC 2487, 1976.
[4] Sannum, H. Heidrun, The First Concrete TLP. The Future Development of the North Sea and
Atlantic Frontier Regions. OCS, Aberdeen 25 and 26 January 1995.
[5] Ruud, M. The Troll Olje Development Project. Vision Eureka, New Technology for Concrete
Structures Offshore. Lillehammer 13 & 16 June 1994
[6] Valenchon, Nagel, Viallon, Belbeoc’h, Rouillon: The NKOSSA concrete oil production barge.
OMAE 1995 – Copenhagen – 14th International conference – June 18-22 1995.
[7] Valenchon, Nagel, Viallon, Belbeoc’h, Rouillon: The NKOSSA concrete oil production barge.
Paper presented at DOT, 30 Oct. / 1 st Nov. 1995, Rio de Janeiro, Brazil.
[8] Sare and Yee Operational experience with pre-stressed concrete barges “Concrete Afloat”,
Proceedings of the conference on concrete ships and floating structures organized by The
Concrete Society in association with the Royal Institution of Naval Architects and held in
London on 3 and 4 March, 1977.
[9] Fjeld (NC), Hall (Phillips), Hoff (Mobil), Michel (Doris), Robberstad (Elf), Vegge (Norw.
Petrol. Directorate), Warland (Statoil): The North Sea concrete platforms – 20 years of
experience, OTC 1994, Houston
[10] Bech, S., Carlsen, J.E.: “Durability of High-Strength Offshore Concrete Structures”.
Proceedings – 5th. International Symposium on Utilisation of High Strength/High Performance
Concrete. Sandefjord, Norway, June 1999.
[11] Derrington, J. A. Prestressed concrete platforms for process plants. Proceedings of the
conference on concrete ships and floating structures organized by The Concrete Society in
association with the Royal Institution of Naval Architects and held in London on 3 and 4
March, 1977.
[12] Morgan, R. G. History of and Experience with Concrete Ships. Proceedings of the conference
on concrete ships and floating structures, Sept. 15-19, 1975 / Berkeley, California, Ben C.
Gerwick jr. Editor.
[13] Nanni, A. and Lista, W.L. Concrete Cracking in Coastal Areas: Problems and Solutions.
Concrete International, Dec. 1988
[14] FIP (Federation Internationale de la Precontrainte) state of the art report: The inspection,
maintenance and repair of concrete sea structures, August 1982
XIV National Conference on Structural Engineering,
Acapulco 2004
Offshore Structures – A new challenge
Knut Sandvik, Rolf Eie and Jan-Diederik Advocaat,
of Aker Kvaerner Engineering & Technology AS, Arnstein Godejord, Kåre O.Hæreid,
Kolbjørn Høyland and Tor Ole Olsen, of Dr.techn.Olav Olsen a.s – Norway
———————-
more studies at:
imulead.com/tolimared/concretesubmarine/anuncios/du/
Kindest Regards,
Wil
concretesubmarine.com
European Submarine Structures AB
A common failure mode of prestressed concrete structures is corrosion of the steel cables used to tension them. The steel is usually treated with materials like wax for corrosion resistance, but if water can work its way to the steel, the tensioners can corrode and the structure can fail. One solution might be to use synthetic fibers such as kevlar or carbon fiber. They would not corrode, but would be much more expensive than steel. Kevlar might be better than carbon fiber due to better abrasion resistance.
Many bridges have been built with prestressed concrete, along with some ships. Many have not failed. Most are not in seawater.
BTW, the MOB design illustrated above looks quite good. It’s more similar to semisubmersibles than seadrome, though both types are related in history. As noted, it won’t work in high sea states.
Once you go away from plate material you can improve structural strength by building curved shapes. This technique is known as thin shell concrete building. It goes very well with the bubble living space concept.
Bringing living space to the ocean at affordable real estate prices will require to go away from classic platforms usual in oil/gas industry and aproach the protective shell concept. Concrete shell structures can be built at a similar cost per cubic meter living space as average real estate prices on land.
Wil
concretesubmarine.com
European Submarine Structures AB
As an alternate building material have you considered featherstone? This is lava rock formed with gas bubbles inside making it lighter than water. One could take inexpensive recycled glass, heat and mix in air to produce a very stable non water errosive filler material in the form of 10 foot cube interlocking blocks. No jacket or covering would be required, it would provide the buoyancy for large structures similar to foam floats used under finger piers and floating docks. One could construct a very large floating surface area by locking the blocks together.
Then using a standard concrete reinforced slab design on top to provide the working surface for buildings and equipment. Instead of using steel for the rebar, use rods made of fiberglass to prevent rotting by the salt. When rebar is subjected to saltwater it rusts and if you have any experience with parking structures you know rust expands to cause spalling of the concrete. As a weight saving measure, all the topper material need not be concrete if the use is not for a weight bearing structure, they have come out with recycled material that is weather resistant used for sidewalks called Terrewalks pavers. http://www.rubbersidewalks.com/pdf/Terrewalks_Flyer.pdf
It has been estimated a family of four requires approx. .1 acres per person (4350 sqft) to grow enough food to be self sufficient, rounding up to 20,000 sqft, a 140 ft x 140 ft square island would be enough provide farm land and housing space with rain water collection to support a modest living area. By locking together 196 of these 10ft cube blocks you could form a stable floating island. These islands could be linked together using the blocks for roads or paths. http://thesietch.org/mysietch/greenspree/2007/07/17/self-sufficiency/